Basic physical parameters of the test soil.

The effect of soil fixation and anti-scour instability of slope vegetation generally depends on the strength and anti-disintegration ability of slope soil due to increase of root system. Therefore, it is particularly necessary to study the disintegration characteristics of expansive soil related to slope instability under acidic conditions (simulated acid rain). In this paper, the response surface method (RSM) was used with the pH value, root diameter, root length, root coefficient, and distribution as independent variables, and the disintegration amount of root-soil (DARS) after 60min as the response value. Then X-ray diffractometer (XRD) was used to analyze the mineral composition changes of the sample under this environment. Simultaneously, the plasticity index of expansive soil at different values of pH was studied to discuss the disintegration mechanism of root compound expansive soil in an acid environments. The results show that the root system improves the anti-disintegration characteristics of the root-soil, and the effects of various factors on the amount of disintegration were as follows: root length > pH value > root distribution > root amount > root diameter. The DARS with a length of 20mm increased by 26.67% and 41.56% compared to the 30mm and 40mm. Compared to the horizontal distribution and horizontal + slant distribution, the DARS with slant distribution was increases by 11.39% and 20.24% respectively. The DARS with 2 roots is increased by 9.92% and 16.75% compared to 4 and 6 roots respectively. The 1mm diameter DARS is 6.65% and 15.49% higher than the 2mm and 3mm, respectively. In addition, an acidic environments can lead to an increase in the amount of disintegration or rate of disintegration. The disintegration at pH = 4.2 was increased by 11.4% and 22.4% compared to pH = 5.6 and pH = 7, respectively. The acidity affects soil disintegration is due to the hydrophilic minerals in the expansive soil react with H+ ion in the acid solution to form soluble salts. Due to the dissociation and leaching of free quartz and metal oxides in the soil to varying degrees, the ability of expansive soil to accumulate is reduced. The intensity of erosion and leaching decreases with increasing pH. In addition, the pH value can affects the plasticity index of the soil, which increases with the increasing pH, thus affects the disintegration properties of the expansive soil.

边坡植被的固土抗冲刷失稳效果，通常取决于根系发育提升的边坡土体强度与抗崩解能力。因此，研究酸性环境（模拟酸雨）下与边坡失稳相关的膨胀土崩解特性尤为必要。本文采用响应面法（Response Surface Method, RSM），以pH值、根径、根长、根系数、根分布作为自变量，以60分钟后根-土复合体崩解量（disintegration amount of root-soil, DARS）作为响应值。随后采用X射线衍射仪（X-ray Diffractometer, XRD）分析该环境下试样的矿物成分变化。同时，研究不同pH条件下膨胀土的塑性指数，以探讨酸性环境中根-土复合膨胀土的崩解机制。研究结果表明，根系可提升根-土复合体的抗崩解特性，各因素对崩解量的影响程度排序为：根长 > pH值 > 根分布 > 根数量 > 根径。根长为20mm的根-土复合体崩解量，较根长30mm与40mm组分别提升26.67%与41.56%。相较于水平分布与水平+斜向分布模式，斜向分布的根-土复合体崩解量分别提升11.39%与20.24%。根系数量为2的组，其崩解量较4根与6根组分别提升9.92%与16.75%。根径为1mm的根-土复合体崩解量，较根径2mm与3mm组分别高出6.65%与15.49%。此外，酸性环境会提升土体崩解量或崩解速率。pH=4.2时的崩解量，较pH=5.6与pH=7组分别提升11.4%与22.4%。酸性环境影响土体崩解的机制在于：膨胀土中的亲水性矿物与酸性溶液中的H+发生反应，生成可溶性盐类。由于土壤中游离石英与金属氧化物发生不同程度的解离与淋溶，膨胀土的团聚能力出现下降。淋溶侵蚀强度随pH值升高而减弱。此外，pH值会影响土体塑性指数，塑性指数随pH值升高而提升，进而影响膨胀土的崩解特性。